The long-range objective of this project is to establish the function (or functions) of the pineal complex in vertebrates. To this end we have engaged in a comparative study for several years. In view of the remarkable diversity of pineal morphology across the vertebrates and implications of involvement of this organ in a great range of physiological mechanisms, including reproduction, thermoregulation, color change, rhythmicity, and ionic regulation, a comparative approach seems clearly warranted. As in the past, we will continue to choose as experimental animals those species which seem most likely to provide answers to questions about pineal functions. Thus, background adaptation and color change will be investigated in fish; ionic regulation will be examined with turtles having salt glands; locomotor rhythms will be studied in salamanders; and temperature regulation will be investigated in mice. In all instances, we have established procedures for manipulating the animals and have demonstrated their potential as suitable experimental subjects for the proposed studies. By investigating pineal organs in non-mammalian as well as in mammalian species, we expect to perceive their truer nature and to define their functional importance in different phylogenetic groups. The pineal complexes of the so-called lower vertebrates are more accessible and amenable to manipulation than are those of mammals. Also, because the pineals of ectotherms are morphologically complex, in contrast to the simple structure found in birds and mammals, they invite curiosity. It is to be expected that through comparative studies of this organ--from photoreceptive types to purely glandular types--new leads can be perceived which will still better explain the mammalian pineal itself.